Using a user defined customization in portal as per this doc https://techdocs.broadcom.com/us/en/vmware-tanzu/bitnami-secure-images/bitnami-secure-images/services/bsi-doc/using-apply-user-defined-customizations-tac-containers.html users can create and upload them to the portal.
Bitnami Secure Images services
When you create a customization one of the steps is to upload it to the Bitnami Secure Images. You may observe the upload getting into "Pending" state and not progressing for a long period of time.
If this occurs please open a support ticket for R&D to review why the upload got stuck.